1. Field of the Invention
The present invention relates to chemical mechanical polishing (CMP) techniques and, more particularly, to the efficient, cost effective, and improved CMP operations.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform chemical mechanical polishing (CMP) operations. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material grows. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then, metal CMP operations are performed to remove excess metallization.
A chemical mechanical polishing (CMP) system is typically utilized to polish a wafer as described above. A CMP system typically includes system components for handling and polishing the surface of a wafer. Such components can be, for example, an orbital polishing pad, or a linear belt polishing pad. The pad itself is typically-made of a polyurethane material or polyurethane in conjunction with other materials such as, for example a stainless steel belt. In operation, the belt pad is put in motion and then a slurry material is applied and spread over the surface of the belt pad. Once the belt pad having slurry on it is moving at a desired rate, the wafer is lowered onto the surface of the belt pad. In this manner, wafer surface that is desired to be planarized is substantially smoothed, much like sandpaper may be used to sand wood. The wafer may then be cleaned in a wafer cleaning system.
FIG. 1A shows a linear polishing apparatus 10 which is typically utilized in a CMP system. The linear polishing apparatus 10 polishes away materials on a surface of a semiconductor wafer 16. The material being removed may be a substrate material of the wafer 16 or one or more layers formed on the wafer 16. Such a layer typically includes one or more of any type of material formed or present during a CMP process such as, for example, dielectric materials, silicon nitride, metals (e.g., aluminum and copper), metal alloys, semiconductor materials, etc. Typically, CMP may be utilized to polish the one or more of the layers on the wafer 16 to planarize a surface layer of the wafer 16.
The linear polishing apparatus 10 utilizes a polishing belt 12 in the prior art, which moves linearly in respect to the surface of the wafer 16. The belt 12 is a continuous belt rotating about rollers (or spindles) 20. The rollers are typically driven by a motor so that the rotational motion of the rollers 20 causes the polishing belt 12 to be driven in a linear motion 22 with respect to the wafer 16. Typically, the polishing belt 12 has seams 14 in different sections of the polishing belt 12.
The wafer 16 is held by a wafer carrier 18. The wafer 16 is typically held in position by mechanical retaining ring and/or by vacuum. The wafer carrier positions the wafer atop the polishing belt 12 so that the surface of the wafer 16 comes in contact with a polishing surface of the polishing belt 12.
FIG. 1B shows a side view of the linear polishing apparatus 10. As discussed above in reference to FIG. 1A, the wafer carrier 18 holds the wafer 16 in position over the polishing belt 12. The polishing belt 12 is a continuous belt typically made up of a polymer material such as, for example, the IC 1000 made by Rodel, Inc. layered upon a supporting layer. The supporting layer is generally made from a firm material such as stainless steel. The polishing belt 12 is rotated by the rollers 20 which drives the polishing belt in the linear motion 22 with respect to the wafer 16. In one example, an air bearing platen 24 supports a section of the polishing belt under the region where the wafer 16 is applied. The platen 24 can then be used to apply air against the under surface of the supporting layer. The applied air thus forms an controllable air bearing that assists in controlling the pressure at which the polishing belt 12 is applied against the surface of the wafer 16. As mentioned, seams 14 of the polishing belt 12 are generally located in several different locations in the polishing belt 12. Therefore, the polishing belt is made up of multiple sheets of a polymer material that are connected together by, for example, an adhesive, stitching, or the like to form a continuous belt. A seam section 30 illustrates one of the seams 14, which will be discussed in greater detail in FIG. 1C. Therefore, during a CMP process, moisture from, for example, slurry may intrude into the inner portion of the polishing belt 12 through the seams 14. The moisture may then attack the adhesive holding the polishing belt and the supporting layer together thus causing delamination of the polishing belt from the supporting layer. Therefore, the prior art designs have serious delamination problems due to moisture intrusion into the seams 14. In addition, shear forces created between the support layer and the polishing belt 12 when moving over the rollers 20 can be a very serious destructive factor and also cause delamination. As a result, the life of the polishing belt may be shortened significantly. Such a shortening of polishing belt life may then cause a considerable decrease in wafer production. This problem is further described in reference to FIG. 1C.
FIG. 1C shows a magnified view of an exemplary seam section 30 after delamination has started to take place. The seam section 30 includes a seam 38, a polymer polishing layer 32 connected on top of a supporting layer 36 by an adhesive 42. Delaminations 40 start to occur between the polymer polishing layer 32 and the supporting layer 36 as the fluids start to attack the integrity of the adhesive material, and thus, the adhesive 42 will either itself start to come off of the supporting layer 36 and/or allow the polishing layer 32 to delaminate progressively as critical CMP operations are in progress. Additionally, when the polymer polishing layer 32 and the supporting layer 42 move over the rollers 20 (as shown in reference to FIG. 1C), shear forces may be created causing serious delaminatory damage.
During a CMP process, slurry is typically applied to the polishing belt 12 of FIGS. 1A and 1B. When this occurs, the moisture from the slurry may seep through the seam 38. In more detail, the delaminations 40 tend to form after continued use of a polishing belt because of the moisture seepage from a surface of the polishing belt down the seam 38 to the adhesive film 42. The moisture seepage can then break down the adhesive film 42. When this occurs, the different layers 32 and 36 of the polishing belt 12 may start to peel off, as described above, due to the loss in adhesion resulting in the delaminations 40. In addition, pressures and shear forces exerted on the polishing belt during the CMP process can serve to exacerbate matters and can greatly increase the creation of the delaminations 40. When the seam section 30 moves over rollers, the support layer 36 does not stretch very much thus defining a neutral axis. The polishing belt 12 on top of the supporting layer 36 typically stretches when it is bending over the roller because outer layers tend to stretch more than inner layers. When the seam section 30 is no longer on the rollers, the stretch disappears and the seam section 30 compresses. This constant stretch and compress cycles tend to create stress in the materials thus creating shear stress between the supporting layer 36 and the polishing belt 12. This shear stress may lead to delamination over time. The delaminations 40 tend to destabilize the polishing pad and significantly reduce the effectiveness and life of the polishing pad. As a result, the polishing pad of the prior art has a reduced life span and therefore wafer production throughput may be drastically reduced due to the time necessary to change the polishing pad. The reduced lifetime of polishing pads also results in the use of more polishing pads by a manufacturer thus incurring even more costs. In addition, if unanticipated delaminations occur, wafers polished by delaminated polishing belts may be defective thus creating further costs for a wafer manufacturer.
As indicated previously, changing pads on a polishing belt may be an extremely expensive and time consuming process. When changing pads, a polishing belt has to typically be sent back to the manufacturer and have the pad stripped from a base belt. This can cause a long period of wafer processing shutdown and can potentially decrease wafer production severely. Therefore, polishing belt structure which breaks down and delaminates after a short period of time may create extreme problems for entities requiring constant and consistent wafer production.
Unfortunately the prior art method and apparatus of CMP operations as described in reference to FIGS. 1A, 1B, and 1C have even more problems. The prior art apparatus also has problems with oxide removal where the topographical nature of the wafers include varying thickness of metallic and dielectric layers such as those found when gaps are formed during the application of such layers. Again, these prior art difficulties arise due to the inability to properly control the polishing pressure applied to the wafer surface due to the lack of cushioning of the polishing pad. Consequently, these problems arise due to the fact that the prior art polishing belt designs do not properly control polishing dynamics because of the lack of cushioning in the polishing pad.
Therefore, there is a need for a method and an apparatus that overcomes the problems of the prior art by having a polishing pad structure that is longer lasting that further enables more consistent and effective polishing in a CMP process.